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Early embryonic specification of vertebrate cranial placodes

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Cranial placodes contribute to many sensory organs and ganglia of the vertebrate head. The olfactory, otic, and lateral line placodes form the sensory receptor cells and neurons of the nose, ear, and lateral line system; the lens placode develops into the lens of the eye; epibranchial, profundal, and trigeminal placodes contribute sensory neurons to cranial nerve ganglia; and the adenohypophyseal placode gives rise to the anterior pituitary, a major endocrine control organ. Despite these differences in fate, all placodes are now known to originate from a common precursor, the preplacodal ectoderm (PPE). The latter is a horseshoe‐shaped domain of ectoderm surrounding the anterior neural plate and neural crest and is defined by expression of transcription factor Six1, its cofactor Eya1, and other members of the Six and Eya families. Studies in zebrafish, Xenopus, and chick reveal that the PPE is specified together with other ectodermal territories (epidermis, neural crest, and neural plate) during early embryogenesis. During gastrulation, domains of ventrally (e.g., Dlx3/Dlx5, GATA2/GATA3, AP2, Msx1, FoxI1, and Vent1/Vent2) and dorsally (e.g., Zic1, Sox3, and Geminin) restricted transcription factors are established in response to a gradient of BMP and help to define non‐neural and neural competence territories, respectively. At neural plate stages, the PPE is then induced in the non‐neural competence territory by signals from the adjacent neural plate and mesoderm including FGF, BMP inhibitors, and Wnt inhibitors. Subsequently, signals from more localized signaling centers induce restricted expression domains of various transcription factors within the PPE, which specify multiplacodal areas and ultimately individual placodes. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Early Embryonic Development > Gastrulation and Neurulation Nervous System Development > Vertebrates: General Principles
Cranial placodes in chick and Xenopus embryos. (a) dorsal view of 10–13 somite‐stage chick embryo. (Modified from Ref . Copyright 2004 Elsevier Ltd). (b) Lateral view of tailbud‐stage Xenopus embryo. (Modified from Ref . Copyright 2000 Wiley Ltd).
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Development of individual placodes from the preplacodal ectoderm (PPE). Posteriorly restricted Wnt signals and signaling centers within the mesoderm and neural plate induce transcription factors within the PPE (red), which specify multiplacodal areas (colored outlines) and individual placodes (colored ovals). Some examples of transcription factors are listed below. Hatched lines indicate signaling events at later (post‐neural plate) stages. Ad, adenohypophyseal placode; ANR, anterior neural ridge; EB, epibranchial placodes; EF, eye field; L, lens placode; LL, lateral line placodes; MHB, midbrain–hindbrain boundary; Not, notochord; Ol, olfactory placode; Ot, otic placode; PP, pharyngeal pouches; Pr/V, profundal/trigeminal placode; R4, rhombomere 4. (Modified from Ref . Copyright 2010 Elsevier Ltd)
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Establishment of ectodermal transcription factor domains in early development. Expression domains of transcription factors are shown as colored lines around schematic cross sections through the cranial region of Xenopus embryos (D, dorsal; V, ventral). Hatched lines refer to downregulation of expression. Transcription factors listed in parentheses only get upregulated in the respectively colored domains during gastrulation (Pax3 and c‐Myc in the lateral part of the turquoise domain; Sox2 in the green domain). The regions destined to give rise to neural plate (NP), neural crest (NC), preplacodal ectoderm (PPE), and epidermis (EP) are shown as fate map for blastula and gastrula stages (a–c: faint colors) and as specified territories for early neurula stages (d: strong colors). Changes in extent and position of the different territories due to gastrulation movements are disregarded here. Signaling activities of the BMP, Wnt, and FGF pathways along the dorsoventral axis are shown by colored lines inside the schematized embryo, with graded activity of BMP and approximate position of sources of BMP inhibitors and Wnt inhibitors indicated (bars; tissue sources are not depicted). Note that many transcription factors become increasingly dorsally (turquoise and green) or ventrally (orange and pink) restricted, and the region of overlap decreases with gastrulation. Neural crest specifiers (blue) and transcription factors defining the PPE (red) become confined to nonoverlapping territories in the neural (turquoise) and non‐neural (orange) ectoderm at the end of gastrulation. See text for details.
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Models of placode specification. (a) The ‘neural plate border state model’ proposes that preplacodal ectoderm (PPE, red) and neural crest (blue) are induced from a common precursor (purple) at the neural plate border, whereas the ‘binary competence model’ proposes that they are induced from non‐neural (yellow) and neural (green) ectodermal competence territories, respectively. (b) Dorsally restricted neural and ventrally restricted non‐neural competence factors overlap during gastrulation (left panel) but then resolve into mutually exclusive territories at the end of gastrulation (middle panel). Inducing signals from adjacent tissues induce PPE (FGF, BMP inhibitors, and Wnt inhibitors; red) and neural crest (FGF, BMP, and Wnt; blue) at the border of non‐neural and neural ectoderm, respectively. (Modified from Ref . Copyright 2006 Elsevier Ltd)
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Development of cranial placodes in Xenopus. (a) Fate map of neural plate‐stage (stage 14) embryo. All placodes arise from a horseshoe‐shaped area around the anterior neural plate, the preplacodal ectoderm (PPE). (b) PPE and neural crest in a neural plate‐stage embryo. (c) After neural fold closure (stage 21), all placodes are closely juxtaposed. Lateral line (LL), otic (Ot), and epibranchial (EB) placodes will arise from a common posterior placodal area (LL/Ot/EB). Neural crest streams are shown as blue broken lines. (d) At tailbud stages (stage 27), placodes have become segregated. Color code matches fate map in (a). (a: Modified from Ref . Copyright 2011 Elsevier Ltd. b–d: Modified from Ref . Copyright 2000 Wiley Ltd and Ref . Copyright 2004 Elsevier Ltd)
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Nervous System Development > Vertebrates: General Principles
Early Embryonic Development > Gastrulation and Neurulation
Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics